Driving circuits for light emitting elements

ABSTRACT

A circuit for driving light emitting elements, such as LEDs, includes a first transistor having a source coupled to ground through a first resistive element, and a second transistor having a gate electrically coupled to a gate of the first transistor, a source electrically coupled to ground, and a drain for electrical connection to a first group of light emitting elements. The circuit also includes circuitry to provide a predetermined voltage at the source of the first transistor, circuitry to compensate for a difference in respective gate-source voltages of the first and second transistors, and circuitry to compensate for a difference in respective drain-source voltages of the first and second transistors. In some implementations, the circuit can achieve relatively low power consumption.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/275,954, filed Oct. 18, 2011, which is incorporated herein in itsentirety.

BACKGROUND

This disclosure relates to circuits for driving light emitting elementssuch as light emitting diodes (LEDs).

LEDs are current-driven devices whose brightness is proportional totheir forward current. Forward current can be controlled in variousways. For example, one technique is to use the LED current-voltage (I-V)curve to determine what voltage needs to be applied to the LED togenerate a desired forward current. Another technique, of regulating LEDcurrent is to drive the LED with a constant-current source. Theconstant-current source can help eliminate changes in current due tovariations in forward voltage, which results in constant LED brightness.In this technique, rather than regulating the output voltage, the inputpower supply regulates the voltage across a current-sense resistor. Thepower supply reference voltage and the value of die current-senseresistor determine the LED current.

One issue that arises in some LED driver circuits is high powerconsumption.

SUMMARY

The subject matter described in this disclosure relates to LED drivercircuits, which in some implementations, can help reduce powerconsumption.

For example, in one aspect, a circuit for driving light emittingelements includes a first transistor having a source coupled to groundthrough a first resistive element, and a second transistor having a gateelectrically coupled to a gate of the first transistor, a sourceelectrically coupled to ground, and a drain for electrical connection toa first group of light emitting elements. The circuit also includescircuitry to provide a predetermined voltage at the source of the firsttransistor, circuitry to compensate for a difference in respectivegate-source voltages of the first and second transistors, and circuitryto compensate for a difference in respective drain-source voltages ofthe first and second transistors.

In a second aspect, a circuit for driving a string of light emittingdiodes includes a first transistor having a gate, a source coupled toground through a first resistive element, and a drain. Circuitry isincluded to provide a voltage having a predetermined value to the sourceof the first transistor. A second transistor has a gate, a sourceelectrically coupled to ground, and a drain for electrical connection tothe string of light emitting diodes. A second resistive element has afirst end coupled electrically to a gate of the first transistor and asecond end coupled electrically to the gate of the second transistor. Afirst current source is coupled electrically between the second end ofthe second resistive element and ground. A third resistive element hasone end coupled electrically to the drain of the first transistor and asecond end coupled electrically to the drain of the. second transistor.

Various apparatus that can include the driving circuits, as well asmethods of operation, are described below.

Some implementations include one or more of the following advantages.For example, as noted above, in some implementations, the circuits canachieve relatively low power consumption. The second transistorgenerates a relatively controlled and stable drive current that, in someimplementations, varies little, if at all, with changes in the voltageof the LED string.

Other aspects, features and advantages will be readily apparent from thefollowing detailed description, the accompanying drawings, and theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagram illustrating driving of multiple LEDstrings.

FIG. 2 illustrate details of an example circuit for driving a single LEDstring.

FIG. 3 is a flow chart of a method of operation of the driving circuit.

FIG. 4 illustrates details of an example circuit for driving multipleLED strings.

FIG. 5 illustrates another example of a circuit for driving multiple LEDstrings.

FIG. 6 illustrates an example of a LED driver circuit with protectioncircuitry.

DETAILED DESCRIPTION

As illustrated in FIG. 1, outputs from a LED driver circuit 10 arecoupled, respectively, to LED strings 11. In the example of FIG. 1,there are six LED strings 11 connected in parallel, each of whichincludes ten LEDs 11A connected in series. In some implementations,however, the driver circuit 10 may drive a different number of LEDstrings (e.g., eight or sixteen) and, in some cases, may drive only asingle LED string. Furthermore, in some implementations, the number ofLEDs in each string 11 may differ from ten.

The number of LED strings, as well as the number of LEDs in each string,may depend on the particular type of device and application. Forexample, the LED driver technology described here can be used, forexample, in backlighting and solid-state lighting applications. Examplesof such, applications include LCD TVs, PC monitors, specialty panels(e.g., in industrial, military, medical, or avionics applications) andgeneral illumination for commercial, residential, industrial andgovernment applications. The LED driver technology described here can beused in other applications as well, including backlighting for varioushandheld devices. The driver circuit 10 can be implemented as anintegrated circuit fabricated, for example, on a silicon or othersemiconductor substrate.

As illustrated in FIG. 1, the driver circuit 10 includes connections toa power supply voltage (VCC) and to ground. The LED strings 11 arecoupled to a LED power supply voltage V_(POWER-LED).

As illustrated in the example of FIG. 2, the driver circuit 10 includesseveral current sources 12, 14, 16, an operational amplifier 18,resistors R1, R2, R3, R4, and transistors M1, M2. The reference currentsource 12 generates a current that flows through resistor R1. Thiscurrent flow generates a reference voltage V1 at the non-inverting input(in+) of the operational amplifier 18. Substantially the same voltage(V1) appears at the inverting input (in−) of the operational amplifier18, and this voltage appears across the resistor R2, which is coupledbetween the source of the transistor MI and ground. Thus, theOperational amplifier 18 regulates the voltage appearing at the sourceof transistor Ml by maintaining the voltage at the inverting input (in−)at the same level as the voltage appearing at the non-inverting input(in+).

As further shown in FIG. 2, the output of the operational amplifier 18is coupled to the gate of transistor M1 and (through resistor R3) to thegate of transistor M2. The transistors M1, M2 can be implemented, forexample, as MOS transistors. In the illustrated example, the size (i.e.,area) of transistor M2, which provides the current for an LED stringcoupled to the drain of transistor M2, is X times larger than the sizeof transistor M1. The value of X can vary over a wide range depending onthe particular circuit design. In some implementations, the, ratio ofthe of the transistors M1:M2 is on the order of about 1:1000. Therelative sizes of the transistors M1, M2 can be used to generate alarger current for the LED string. For example; if the gate-sourcevoltage's (Vgs) of the transistors M1, M2 were substantially the same,then transistor M2 would provide a controllable, substantially stablecurrent that is about X times as large as the current through transistorM1. However, in actual implementations, the gate-source voltages on thetransistors differ from one another due to the fact that the source ofthe transistor. M2 is connected directly to ground, whereas the sourceof the transistor M1 is connected to ground through resistor R2. Withoutadditional circuit components such as those described below (e.g.,resistors R3, R4 and current source 16), the current generated bytransistor M2 will typically depend on the voltage of the LED stringbecause of the difference in the gate-source voltages. Thus, in theabsence of the additional circuit components (e.g., resistors R3, R4 andcurrent source 16), the current generated by transistor M2 for the LEDstring will vary and, thus, is not well-controlled or stable.

To help ensure that the current generated by second transistor M2remains at the desired level, additional circuit components (e.g.,resistors R3, R4 and current source 16) are provided to compensate fordifferences in the gate-source voltages of the transistors M1, M2 and tocompensate for differences in their drain-source voltages.

To compensate for the difference in the gate-source voltages of thetransistors M1, M2, resistor R3 is coupled between the gates of thetransistors M1, M2. In addition, a current source 16 is coupled betweenthe gate of transistor M2 and ground. The values of the resistor R3 andthe current source 16 should be selected such that the voltage V1 acrossresistor R2 is substantially equal to the value of the resistor R3multiplied by the current I₃ generated by the current source 16 (i.e.,V1=I₃×R3). The voltage generated by the current I₃ (from source 16)flowing through resistor R3 compensates for the difference ingate-source voltages of the transistors M1, M2. Furthermore, tocompensate for the difference in drain-source voltages (Vds) of thetransistors M1, M2, resistor R4 is coupled between the respective drainsof the transistors.

As indicated by FIG. 3, in operation, the circuit 10 provides apredetermined voltage at the source of the first transistor M1, (102),compensates for a difference in respective gate-source voltages of thefirst and second transistors M1, M2 (104), and compensates for adifference in respective drain-source voltages of the first and secondtransistors M1, M2 (106).

As an illustrative example, it is assumed that the values of resistorsR2, R3 and R4 are the same. In that case, half the current from thecurrent source 14 flows through transistor M1 and resistor R2, and thesame amount of current flows through resistor R4. Thus, in this example,a current I₂/2 flows through transistor M1 (and resistor R2). Likewise,when the voltage of the LED string is lower than the power supplyvoltage (VCC), a current I₂/2 also flows through resistor, R4 tocompensate for the difference in drain-source voltages between thetransistors M1 and M2.

Continuing with the foregoing example, the voltage V1 at the source ofthe transistor M1 is equal to the product of the resistance R2 and thecurrent flowing through that resistor (i.e., V1=I₂/2×R2). The voltage V1also is equal to the product of the current from current source 12 andthe resistance R1 (i.e., V1=I₁×R1). Values of the current sources 12, 14and the resistors R1, R2 can be selected using the foregoinginformation.

As explained above, the values of the resistor R3 and the third currentsource 16 are selected such that V1=I₃×R3. Using the foregoing examplein which R3=R2, the value of the current source would be set equal toI₂/2 so as to compensate for the difference in gate-source voltages ofthe transistors M1 and M2.

In some implementations, the values of the resistors and current sourcesmay differ from the foregoing example.

By using the driver circuit 10 of FIG. 2, the current generated bytransistor M2 can be substantially independent of the voltage of the LEDstring. The circuit 10 can, therefore, provide a more controllable drivecurrent.

The extent of power savings that can be achieved in some implementationscan be appreciated by considering a driver circuit without transistorcurrent sources 14, 16 and resistors R3, R4, but with the drain oftransistor M1 coupled to the LED string. If V1 were 250 mV and thecurrent required of transistor M1 were 60 mA, the power consumptionwould be on the order of 0.015 Watts. If there are eight LED strings inthe device, power consumption would be on the order of 0.12 Watts. Therequirement of a voltage and current on resistor R2 results insignificant waste or loss of power. In contrast, the driver circuit 10of FIG. 2 can achieve a significant, reduction in power consumption, forexample, on the order of 99% in some implementations.

Furthermore, the drive circuit 10 of. FIG. 2 can result in a significantreduction in the amount of die area. For a driver circuit withouttransistor M2, current sources 14, 16 and resistors R3, R4, but with thedrain of transistor M1 Coupled to the LED string, the ratio of R1:R2 mayneed to be on the order of 1,000 for some implementations, which canerequire a large die area for resistor R2. In contrast, the drivercircuit of FIG. 2 does not require such a high ratio of resistor valuesand, therefore, can significantly reduce the amount of die area required(e.g., by as much as about 20% for some implementations).

FIGS. 4 and 5 illustrate examples of circuits for driving multiple LEDstrings. If PWM control of the respective LED strings is to besubstantially the same (e.g., same-phase and frequency), the circuit 20of FIG. 4 can be used. Circuit 20 is similar to circuit 10 of FIG. 2except that an additional transistor M3 is provided to generate thecurrent for the second LED string. As illustrated in the example of FIG.4, the gate of transistor M3 is coupled to the gate of transistor M2,which is coupled to the gate of transistor M1 through resistor R3 asdescribed above. The drain of transistor M2 is coupled to the first LEDstring, whereas the drain of transistor M3 is coupled to the second LEDstring. The source of transistor M3, like the source of transistor M2 iscoupled directly to ground. In this example, the size of transistor M3can be substantially the same as the size of transistor M2.

On the other hand, if PWM control of the respective LED strings is todiffer from one another, then the circuit 30 of FIG. 5 can be used.Different LED strings may require different currents, for example, ifthe strings contain, different types of LEDs (e.g., the first stringcontains LEDs that emit light of a first color, and the second stringcontains LEDs that emit light of a second color, different from thefirst color). The circuit 30 of FIG. 5 includes multiple copies (in thiscase two) of the circuit 10 of FIG. 2. Each circuit 10 is coupled to oneof the LED strings.

Although FIGS. 4 and 5 illustrate only two LED strings, someimplementations may include a greater number of LED strings. In thatcase, additional circuitry can be added as needed. For example, in FIG.4, additional transistors similar to M2 and M3 can be provided togenerate the current needed to drive the additional LED strings.Likewise, in FIG. 5, additional copies of the circuit 10 can be providedto generate the current needed to drive the additional LED strings.

FIG. 6 illustrates a drive circuit 40 that is similar to the circuit ofFIG. 2, but which also includes protection diode 42 or other circuitcomponents to protect the current source 14 in the event that thevoltage of the LED string becomes greater than the power supply voltageVCC. Instead of the diode 42, other circuit components can be used, suchas a clamp. Furthermore, the protection circuitry can be separate fromthe current source 14 or can be part of the current source 14.

Each, resistive element R1, R2, R3, R4 can be implemented, respectively,for example, as a single resistive component or as a combination ofresistive components connected in series and/or in parallel.

Other implementations are within the scope of the claims

What is claimed is:
 1. A circuit for driving a string of light emittingdiodes, the circuit comprising: a first MOS transistor; a second MOStransistor including a drain for electrical coupling to a first stringof light emitting diodes and including a source coupled to ground; afirst resistive element electrically coupling a source of the first MOStransistor to ground; circuitry, comprising an operational amplifier,operable to regulate a voltage across the first resistive element, theoperational amplifier including an inverting input electrically coupledto the source of the first MOS transistor, and an output electricallycoupled to the gate of the first MOS transistor; a second resistiveelement having a first end electrically coupled to a gate of the firstMOS transistor and having a second end electrically coupled to a gate ofthe second MOS transistor; a first current source electrically coupledbetween the gate of the second MOS transistor and ground; a secondcurrent source electrically coupled between a drain of the first MOStransistor and a power supply voltage; and a third resistive elementelectrically coupled between the drain of the first MOS transistor and adrain of the second MOS transistor.
 2. The circuit of claim 1 whereinthe operational amplifier has a non-inverting input electrically coupledto a predetermined voltage, and has an inverting input electricallycoupled to a node between the source of the first MOS transistor and thefirst resistive element.
 3. The circuit of claim 2 wherein the circuitryto regulate the voltage across the first resistive element furtherincludes: a third current source; and a fourth resistive elementelectrically coupled between the third current source and ground; andwherein the non-inverting input of the operational amplifier iselectrically coupled to a node between the third current source and thefourth resistive element, and wherein the inverting input of theoperational amplifier is electrically coupled to the node between thesource of the first MOS transistor and the first resistive element. 4.The circuit of claim 1 further comprising a third MOS transistorincluding a drain for electrical coupling to a second string of lightemitting diodes, a gate electrically coupled to the gate of the secondMOS transistor, and a source electrically coupled to ground.
 5. Thecircuit of claim 1 further comprising circuitry, including a diode,arranged to protect the second current source when a voltage across thestring of light emitting diodes is greater than the power supplyvoltage.
 6. The circuit of claim 5 wherein the diode is coupled betweenthe second current source and the drain of the first MOS transistor. 7.An apparatus comprising: a first group of light emitting diodes inseries; and a circuit for driving the first group of light emittingdiodes, wherein the circuit includes: a first MOS transistor; a firstresistive element electrically coupling a source of the first MOStransistor to ground; a second MOS transistor including a drainelectrically coupled to the first group of light emitting diodes andincluding a source coupled to ground; voltage regulating circuitry,comprising an operational amplifier, the voltage regulating circuitryarranged to regulate a voltage across the first resistive element; asecond resistive element electrically coupled between a gate of thefirst MOS transistor and a gate of the second MOS transistor; a firstcurrent source electrically coupled between the gate of the second MOStransistor and ground; a second current source electrically coupledbetween a drain of the first MOS transistor and a power supply voltage;and a third resistive element electrically coupled between the drain ofthe first MOS transistor and a drain of the second MOS transistor. 8.The apparatus of claim 7 wherein a current generated by the second MOStransistor is substantially independent of a voltage across the firstgroup of light emitting diodes.
 9. The apparatus of claim 7 furthercomprising: a second group of light emitting diodes in series; and athird MOS transistor including a drain electrically coupled to thesecond group of light emitting diodes, a gate electrically coupled tothe gate of the second MOS transistor, and a source electrically coupledto ground.
 10. The apparatus of claim 7 further comprising: a pluralityof additional groups of light emitting diodes; a plurality of additionalMOS transistors each of which includes a drain electrically coupled to arespective one of the additional groups of light emitting diodes, a gateelectrically coupled to the gate of the second MOS transistor, and asource electrically coupled to ground.
 11. The apparatus of claim 7wherein the operational amplifier has a non-inverting input electricallycoupled to a predetermined voltage, and has an inverting inputelectrically coupled to a node between the source of the first MOStransistor and the first resistive element.
 12. The apparatus of claim 7further comprising circuitry, including a diode, arranged to protect thesecond current source when a voltage across the first group of lightemitting diodes is greater than the power supply voltage.
 13. Theapparatus of claim 11 wherein the diode is coupled between the secondcurrent source and the drain of the first MOS transistor.
 14. A circuitfor driving a group of light emitting elements in series, the circuitcomprising: a first transistor; a first resistive element electricallycoupling a source of the first transistor to ground; a second transistorincluding a drain arranged to provide an output for driving the group oflight emitting elements and including a source coupled to ground;circuitry, comprising an operational amplifier, operable to regulate avoltage across the first resistive element, wherein the operationalamplifier includes an inverting input electrically coupled to the sourceof the first transistor, and an output electrically coupled to the gateof the first transistor; a second resistive element electrically coupledbetween a gate of the first transistor and a gate of the secondtransistor; a first current source electrically coupled between the gateof the second transistor and ground a second current source electricallycoupled between a drain of the first transistor and a power supplyvoltage; and a third resistive element electrically coupled between thedrain of the first transistor and a drain of the second transistor. 15.The circuit of claim 14 wherein a current generated by the secondtransistor is substantially independent of a voltage across the group oflight emitting elements.
 16. The circuit of claim 1 wherein the sourceof the second MOS transistor is connected directly to ground.
 17. Thecircuit of claim 1 operable such that a voltage across the firstresistive element is substantially equal to a product of currentgenerated by the first current source and a value of the secondresistive element, and wherein, collectively, the second and thirdresistive elements and the first current source are operable such thatvoltage generated by current flowing through the first resistive elementcompensates for a difference in gate-source voltages of the first andsecond MOS transistors, and are operable to compensate for a differencein drain-source voltages of the first and second MOS transistors. 18.The circuit of claim 7 operable such that, during operation of thecircuit, a voltage across the first resistive element is substantiallyequal to a product of current generated by the first current source anda value of the second resistive element.